Acoustic wave filter



Nov. 20, 1928.

1,692,317 a. w. STEWART v ACOUSTIC WAVE FILTER Original Filed July 25,1922 7Fvaslm/lad Eneryy H a mum i i mm Him 10 0/ Frefdmcy Patented N...20, 1928.

UNITED STATES PATENT o en.

GEORGE WALTER STEWART, OF IOWA. CITY, ICWIT-A, ASSIGNOB TO AMERTCN TELE-PHONE AND TELEGRAPH COMPANY, A CORPORATION OF NEW YORK. i I

ACOUSTIC WAVE FILTER.

Application filed m 25, mafs erial no. 577,409. ews September 1a, 1928.

This invention relates to an acoustic wave filter constructed totransmit without serious diminution sinusoidal waves of all frequencieslying within a range or' ranges of 5 preassigned' limiting frequencieswhile attenuating and approximately extinguishing sinusoidal acousticwaves of frequenc1es lying outside the limits of the preassigned rangeor ranges.

The nature of the invention can be best presented by the use of theterm, acoustic impedance .which will be defined. In a column of gasconfined in a tube, consider a portion which is short compared with thewave length of the acoustic sinusoidal wave passing through the gas inthe tube. There will be a sinusoidally varying pressure differenceacting upon the gas and a resulting sinusoidal variation in rate ofchange of volume dis- 0 placement. These two sinusoidal variations willnot be in the same phase. If they are expressed in the well-knowncomplex notation, then their ratio will also be a complex quantity. Thevalue of the acoustic imped- 2 ance in this complex notation is thecomplex ratio of the pressure difference applied to the rate of changeof volume displacement; The absolute value of the acoustic impedance istherefore the ratio of the maximum difference of pressure applied to themaximum rate of change of volume displacement. The acoustic wave maypass through the material or it. may not as in the case of a confinedvolume of gas with one opening through which the gas vibrates. In Viewof the above definition, acoustic impedance is a term that can beapplied to any vibrating portion ofany material or medium whatsoever andis therefore the most convenient term to use in describing the basis ofthe invention.

By my definition, the value of the acoustic impedance is a ratio betweentwo values, yet it is convenient to refer to a portion of a medium aspossessing impedance and, indeed,

to refer to that portion as an impedance. This is analogous to the usageof the terms resistance, inductance, etc., in electrical literature.(For reference to acoustical impedance in acoustical literature seeAcousto tie impedance and its measurementfby A. E. KennellyandK.Kurokawa, Proc. Am. Acad. of Arts and Sciences, vol. 56, No. 1, p. 3,

1921; and bibliography there given, and also to Acoustical impedance,and thetheory of ments has important applications in all RCOIlStlOdevices whether receiving or trans- 'quencies without seriouslydiminishing the flow of acoustlc ener 'y in other frequencies.

limits being also preassigned, the acoustic tically zero in thefrequencies intendedvto be horns and of the phonograph by Arthur Gordon\Vebster, National Academy of Sciences, vol. 5-, pp). 275-282, July,1919.) It IS further to be 0 served that, since impedances may be added,one or more'when' joined 1n series or in parallel may be regarded as asingle impedance.

My invention, though it may find expresslon 1n man embodiments hascommon to all the broad ,.i ea ofa wave filter in the nature of anacoustically conducting medium or condult comprising a seriesof acousticimpedanccs along or through which the Waves are transmitted to thedesired point, and at unction points between these elements ofimpedance, branches each .containing an acoustic impedance, the valuesof all these acoustic impedance .elements' being so' proportioned thatthe conduit will transmit with small diminution sinusoidal. acousticwaves of all frequencies lying within specified and preasslgned limitsor ranges and markedly extinguishing waves of all frequencies lyingoutside these limits.

My invention in one or more of its embodimittin wherein it is desired toeliminate to a sens ble or to a marked degree the flow of acousticenergy in specified groups of fre- .85

Illustrations would the sensible or the marked diminution intransmission of frequencies above a specified limit in the rendition ofphonograph records, in the making of them, and in transmission totelephonic transmitters or from telephonic receivers. In

fact, in any equipment whatsoever wherein acoustic transmission can beimproved by the sensible or the marked diminution of the group offrequencies existing above a specified limit, below a specified limit,simultane-. ously both above and below specified limits, or'finallybetween specified limits, all these wave filter, in one or more ofitsrforms has an important and 'a uniqueapplication. Uniqueness of theacoustic wave filter rests in its ability to make the transmissionpraceliminated without a. serious diminution in the frequencies forwhich transmission is desired, in its ability to modify the degree ofelimination and also in the fact that the frequentl limits can bepreassigned.

For a comprehension of the prmciples upon which the invention is basedand the application of such principles I make reference to theaccompanying drawings which illustrate three typical forms of acousticalwave filters and their characteristics. 7 Fig. 1 is an elevation, partlyin section of an acoustic wave filter according to the invention, whichis representative of the types available for transmitting waves offrequencies lying between two preassigned limits while attenuating wavesof frequencles lying outside the fixed range.

Fig. 2 is a similar view of a filter for transmitting waves offrequencies between zero and a predetermined limiting value.

Fig. 3 is a similar view of a filter for transmitting waves offrequencies above a certain preassigned value.

Fig. 4 is a. diagrammatic representation of an acoustical path includingimpedances in the path and in branches laterally thereof, for

facilitatin the understanding of the theory on which t e invention isbased; and

Figs. 5, 6 and 7 are diagrams showing the characteristics of the wavefilters illustrated in Figs 1, 2 and 3 respectively.

The form of the invention illustrated by Fig. 1 consists of a series ofsections containin e ual volumes V and similafl branch -tu es, F and H.At the lower terminus of .each of these tubes there is an opening into Vadjacent to an opening E into A G, which latter is the conduit orconducting line of the acoustic waves to be transmitted. The uppertermini of D, F and H are in the undisturbed acoustic medium, which inthe illustration, is a gas. The openings F5 are preferably uniformlspaced. The elements of acoustical im edzmce referred to above will nowbe descrlbed. In the conduit, A G, the portion of the gas between twoconsecutive openings E possesses impedance and may be properly called animpedance. The impedances in series are then, these two impedancesbetween the branch openings in the conduit. The impedance in the branchiscomposed of two parts, one an enclosed volume of gas with an orificeand one a volume of gas terminating in the surrounding medium,'which isassumed undisturbed. These two in arallel form the acoustical impedancein t e side branch. This filter, as will be later pointed out, permitsfrequencies between two preassigned limits to pass but highly attenuatesall other neighboring frequencles.

The form of the invention illustrated by Fig. 2 is essentiall similar tothat in Fig. 1, exce t that the tu ular channels to the outside ave beenremoved, leaving the volume V the only branchimpedance. The conduit A Gis composed of two telescoping tubes R and T. The tube R has a pluralityof circular series of openin 0 while the tube T has a plurality of dou1e rows P P The tubes may be adjusted relative to each other to bringthe series 0 in register with either the rows P or P to vary the numberof openings from the conduit into the branches, The elements ofimpedances in series in A Gr are found between the openings into oneside chamber and the openings into the next chamher, just as in Fig. 1.The branch impedances are the equal volumes V with openings into AG.This filter, Fig. 2 will permit all frequencies between zero and apreassigned value to ass and will prevent all neighboring frequenclesabove this value from being transmitted. It is called alow-frequency-pass acousticfilter.

The form of the invention illustrated by Fig. 3, is similar to that inFig. 1, exce t that the side column of gas is made the on y art of thebranch, the enclosed volume, V,, aving been removed. The impedances inseries in th'conduit A Gr are just as before and the branch impedancesare now composed only of the columns defined by the branch tubes Fextending outwardly from openings E in the conduit A G This filter willtransmit through the conduit all frequencies above a certain preassignedvalue and will markedly attenuate all other frequencies.

It should be noted that the material used in construction in any one ofthese filters is not an essential feature, and-that any of them can bereadily built by one skilled in the art. In fact the walls serve thesole function of preventing the'cross transmission of waves or ofacoustically enclosing the vibrating medium, which in the drawings, is agas. Any medium acoustically enclosed in a similar configuration wouldgive similar transmitting and attenuating characteristics, as theaccompanying theory will show.

It should be clearly understood at the very outset that my inventiondiffers fundamentally both in structure and principle from randomopenings in acoustic conduits through which sound may pass. The effectis produced not by absorption of energy and dissipation, notb thewell-known phenomenon of resonance ut by'the reactions and interactionsof similar sections producing not dissipation but refusal totransmission. It is an interference phenomenon.

The fundamental principles underlying myinvention and the manner ofapplying the same so as to provide a structure embodying the inventionwill now be set forth. For the purpose of deriving the mathematicalformulae pertaining to the theory of my in- .125

mission is ne ligible-of in an otherwise undisturbed region of anacoustic medium. Let

to acoustic sinusoidal waves of a frequency be transmitted throughconduit A. G E G, which is for the purposes of the theory, assumed to'bea portion of a structure of infinite length. Assume the nature of Z, tobe such that there is a common constant pressure at or near the terminiof these branch impedances. Let Inl represent the rate of change ofvolume displacement in the con-, duit in the n section, I being complex.If now it is assumed that the algebraic sum of the Is at any junctionpoint'is zero and apply the definition of impedance, we have theollowing equation:

2 n-1 n) 2 n"' n+1) Jp If AP is the (complex) pressure difference over abranch,

wherein Zoo is the impedance of the infinite network to the right in thefigure of the section considered and therefore has the same value inboth equations.

in (2) and dividing we have,

Thus the ratio of successive Is is constant,

but, in general, complex. Let its value be 6". Substituting this valuein (1) we have, Z Y

From (4), if Y is a pure imaginary, the rate of volume displacements intwo adjacent sections differ only in phase, that is, there'is noattenuation. If Y is not a pure imaginary,

and hence the limiting values of no attenua -tion are determined by thefollowing: 4 v

If the actual values of Z and Z are substi Substituting the tuted in (7)and (8), the limitingvalues of frequencies for no attenuation are found.These considerations show that an acoustic wave filter having regions ofattenuation and no attenuation can be constructed if the conduit andbranches are composed of acoustic impedances,-the magnitudes of whichvary in difi'erent manners with frequency, sothat their ratio passesthrough the range of values between zero and 4. I v

It can be shown that if two acoustic imwherein M= g g and and Ma is themass of the branch a, is the stifi'nessof branch 6, So and Sb are theareas of branches at and I) respectively. f is herein defined-by theequality x Pe X being the volume displacement. But in accordance withthe definition of impedance, the impedance of the combined branches inparallel is, from (9), p

I Similarly it can be shown that if these two acoustic impedances, M andC, areconnected in series the resulting impedanceis,

' Further, if two acoustic impedances in series,

M 1 andC areconnected in parallel with another Impedance M theresultingimpedance is,

We shall hereinafter referto the impedances as above defined, as aninertance, and to C as a'capacitance. V

In securing a practical construction'we will" assume that a piece ofconduit or conducting material, short in, comparison with wave length,is equivalent to an inertance and a capacitance connected in paralleL Ifthe conduit is cylindrical M, fromi definition, becomes where, oisthedensityof the medium, Zthe length, and S the area ofcrosssection. assumea volume of gas, .V ,"aco'ustically en- In order to find the 'valueofC,

closed except for a small opening. At any instant, =6p, where dip is theexcess pressure acting and X is the volume placement. But, fromfundamental considerations, 6p= a where a is the velocity of sound and Vis the volume of the medium.

'We shall also assume that iLM} is the inertance of an orifice orchannel it is synonymous with /K, where K is the con- I ductivity (seeRayleigh, Theory of Sound,

' ,vol. II, p. 172, 1896, Equation (1 and p. 181,

' ting acoustic waves say from A to G branch M and V in series connectedEquation (1) of the channel andence,

wherein Z is the length of the circular channel and r its radius.

The application of these formulae in a structure and the method by whichanyone skilled in the art may construct a wave filter which willtransmit with small attenuation a definite, preassigned' band offrequencies while attenuating all outlying frequencies, will now beshown.

In Fig. 1, the cylindrical volume of gas is the acoustic conductor orconduit, transmt- (referring to Fig. 4 is the volume between 1 and theadjacent junction. Its M is denoted by M and its ,G by C Z consists ofthe in parallel with another branch, a tube E F or M M; is the inertanceof the orifice of V and M the inertance of the tube. v For the sake ofsimplicity we will assume that we may neglect C and with this provisionand the application of (10) as Z,, and (12) as Z we have from conditions(7 and (8) respectively, the following:

I =l T 1 27! C2(M2 1 M +4M (15) 2 271' C m hlg If now we substitute thevalues for B1 and M p-i and p g: respectively, for 0 d for M; the valuein (13), we will have the limiting frequencies expressed in terms of thedimensions of the apparatus and the velocity of sound. Thus anyoneskilled in the art can construct such a filter having a singletransmission band with f and f, the limiting fre uencies ,thereof.Outside of the region of equencies from f, to f. there will be anattenuation.

In Fig. 2 the wave filter may be regarded as a modification of Fig. 1,by the removal of the branch tube. This is the same in effect as makingM co. (14) and .(15) therefore become H/ O (M +4M;) It is thus possible'to construct alow frequercy-pass' filter, which will transmit all thosefrequencies from zero up to the value, f and attenuate those above thisFormula (17) ,-when the foregoing values of 0 M and M, are substituted,will enable anyone skilled in the art to construct a wave filter whichwill have the characteristics just stated, the value of f beingpreassigned.

In Fig. 3, we have a construction similar to Fig. 1 except that V andthe opening into ithave been removed. Heretofore we have assumed G =0and have considered the inertance of the tube M only, this proving to bein fair agreement with experiment. In Fig. 3, however, we are dealingwith only inertance in the branch tube and hence C, now assumes animportance. Then-Z remains as in (10) and Z becomes from (10)Substituting these values in (7) and (8) we have,

Z M may be expressed por as an ounce ac- 2 cording to (13), the latterbeingadopted if Z is short. C By substituting these values we havesecured a value of f in terms of the dimensions and of the velocity ofsound and thus anyone skilled in the art is the number of sections usedand upon the relative values of wave length and length of a section. 1It is to be observed that in (14) f can be modified by the change inorifice, or'M", and that in (15 and (17), f can be modified in a similarmanner. Thus adjustable filters can be made either by providing for analteration in the number of orificesas in Fig. 2 or by changing the sizeof one orifice.

It is to be understood that the general theory including (7 and (8)should be regarded as correct within the limits of the approximationsused, but that in the derive; tion of later equations the assumptionsare somewhat empirical and hence lead to equations semi-empirical incharacter but serviceable in actual filter construction..Notwithstanding the approximations all are, clearly dependent upon thegeneral theory which is basic. Thus, though the exact forms of thesepractical equations are not essential in the application of theprinciples herein set forth,

they do aid one skilled in the art to construct a filter that will haveapproximately thepreassigned values of f, and f It is also understoodthat in the successful design and construction of filters by theseformulae regard will be taken of the original assumptions that thejunctions are virtuall points, making the algebraic sum of the I s zero,and that the length of a section is small compared to a wave length. Asthe formulae do not hold for relatively short wave lengths, additionaltransmission bands may pass filters such as shown in Figs. 1 and 2,which bands cannot be predicated by the formulae given. Such additionaland relatively high frequency bandsmay be determined empirically or maybe redicted by theory and formulae which are ased upon assumptionsadditional to those specified above.

It will be further understood that the number of sections of the wavefilter needed will depend onthe degree desired for the attenuation ofthe frequencies to be filtered out. This control can also be obtained bymodifying the size of the opening into the side branches, and by theinsertion of a medium of difl'erent material in the openings from theconduit into the branches. Both methods produce a diminution in thevibrations in the branches and thus decrease the filtering action assuch. 7

It should further be observed that though the theory provides for theuse of but one medium in the acoustically enclosed region, it can bereadily extended to include any number of media by the observation thatthese would merely introduce additional reflections at the surfaces ofseparation and thereby decrease the eifect herein described. As anexample may be cited the insertion of a thin diaphragm across theopenings into the side branches. These diaphragmsreflect the acousticwaves, transmitting only a fraction of the amplitude of vibration whichwould occur without the diaphragms.

By reference to the theory it can be seen that, in em; it Yis-imaginary, orif the frequency isin the unattenuatedregion,

there is a definite change of base in passing from one section of-theconuit to the next. Outslde this region of frequencies Y is complex andhence, according to the theory, there It should be further understoodthat the performance of such filters shows that it is easy to produce inthe regions of attenuation a transmission that is sensibly zero. Fig. 5,

Fig. 6 and- Fig. 7 show the nature of the 2 transmission obtainedexperimentally by thredfilters, one of each type. The contrast intransmission in the attenuated and nonattenuated regions is unique inacoustics. As

an illustration of the application of an acoustic wave filter, thephonograph will be cited. The filter may be applied anywhere in theconduit from the sound box to the broad flaring portion, but it maybe'readily applied by inserting the filter in the tube between the soundbox or diaphragm and the crock. If the filter is that shown in Fig. 2,giving a transmission curve like that in Fig.

6, the efiect upon the music is marked. The

undesirable high frequencies are removed in the rendition of the musicgiving a more natural and more pleasing efiect and simultaneouslyremoving much of the harshness of the scratching sound.

- In the foregoing I have evolved basic formulae for the practicalconstruction of wave filters, adapted to attenuate sound waves ofcertain frequencies while'permitting sound waves of difierentfrequencies to pass. substantially unattenuated. It is needless to saythat within the scope of latitude of initially selecting one ofthevariable quantities inthe "formulae as a starting point and evalu' atingthe others from the formulae, the numher. of specific constructionspossible is in finite. I

Although the diagrams illustrate the performance of filters which effecta complete suppression or extinction of ranges of frequencies, it isobvious that the invention is not limited to the provision of filterswhich effect a complete suppression of a range of frequencies.Theinvention may be applied to eifect any desired degree of attenuationof a predetermined range or ranges 'of frequencies, the degree ofattenuatlon depending upon the particular problem involved and varyingfrom a relatively slight attenuation to complete suppression.

As has been pointed out, the invention has as its cardinalpoin't thecorrelation of acoustic impedances in series and forming part of anacoustic'line of transmission and acoustic impedances in lateralbranches or in shunt,

as it were, with the impedances in series, in close analogy to anelectric circuit containing impedances in series with the line and inpar allel or shunt with the line.

This application is intended as a continuation in part of my applicationSer. No. 430,999, filed December 15, 1920.

, I claim:

1. An acoustic wave filter, comprising a sound-conducting mediumdefining a soundtransmitting path and means defining an acousticimpedance so proportioned and-arranged that waves of frequencies lyingwithin a predetermined range are considerably more attenuated in passingalong the said path than waves of frequencies lying within anotherrange.

2. An acoustic wave filter, comprising means acoustically confining afluid soundconducting medium to define a sound-transmiting path andmeans defining an acoustic impedance so -proportioned and arranged thatwaves of frequencies lying within a predetermined range are considerablymore attenuated in passing through the said path than waves offrequencies lying within another range.

3. An acoustic wave filter, comprising a sound-conducting mediumdefining a soundtransmitting path and means defining an acousticimpedance so proportioned and arranged that waves of frequencies lyingwithin a predetermined range are transmitted along said path withoutserious attenuation and that waves of frequencies lying outside saidrange are materially attenuated.

4. An acoustic wave filter, comprising means acoustically confining afluid soundconducting medium to define a sound-transmitting path andmeans defining an acoustic impedance so proportioned and arranged thatwaves of frequencies lying within a predetermined range are transmittedthrough said path without serious attenuation and that waves offrequencies lying outside said range are materially attenuated.

5. An acoustic wave filter, comprising a sound-conducting mediumdefining a soundtrarsmitting path and means defining an acousticimpedance so proportioned and arranged that waves of frequencies lyingwithin a predetermined ran e are transmitted along said pathsubstantially without attenuation and thatwaves of frequencies lyingoutside such range are materially attenuated.

6. An acoustic wave filter, comprising means acoustically confining asound-com ducting medium to define a sound-transmitting path and meansdefining an acoustic impedance so proportioned and arranged that wavesof frequencies lying within a predetermined range are transmlttedthrough said path substantially without attenuation and that waves offrequencies distinctly outside 7. An acoustic wave filter, comprisingmeans defining a plurality of acoustic im-I pedances in series formingpart of an acoustic path and a plurality of impedances branchingtherefrom, said impedances being so proportioned and arranged that wavesof frequencies lying within a predetermined range are transmitted alongthe said path without serious attenuation and that waves offrequencieslying outside such range are materially attenuated.

8. An acoustic wave filter, comprising means composed of a plurality ofsections constituting a sound transmitting path, each section includinga, mass of the said medium and a volume of the medium disposed branchingtherefrom, said mass and volume being so proportioned that waves offrequencies lying within a predetermined range are transmitted along thesaid path without serious attenuation and that waves of frequencieslying outside such range are materially attenuated. 9. An acoustic .wavefilter, comprising means acoustically confining a fluid sound- 4conducting medium to definea plurality of sections constituting asound-transmitting path, each section including a mass of the saidmedium and a volume of the medium disposed branching therefrom, saidmass and volume being so proportioned that waves of frequencies aretransmitted along the said path without serious attenuation and thatwaves of frequencies lying outside such range I are materiallyattenuated.

10. An acoustic wave filter, comprising means defining an acoustic pathand including acoustic impedances disposed at regular intervals in thedirection of length of the path, said impedances being so proportionedand arranged that waves of frequencies lying within a predeterminedrange are transmitted along the path without serious attenuation andthat waves of frequencies lying outside such range are materiallyattenuated.

11. An acoustic wave filter, comprising means constituting an acousticpath and ineluding acoustic impedances in series in the path andimpedances branching from and alternating with the impedances in series,said impedances being so proportioned'that due to their interaction andreaction upon each other, waves of frequencies lying within apredetermined range are transmitted through the path without seriousattenuation and that waves of frequencies lying outside such range arematerially attenuated.

12. An acoustic wave filter comprising means defining a plurality ofacoustic impedance elements in series and a plurality of acousticimpedance elements branching therefrom at the junction points in theseries, said impedances being so proportioned and arranged that waves offrequencies lying within a predetermined range are transmitted subofsuch range are substantially extinguished. stantially withoutattenuation and that waves acoustic enclosure surrounding each openingoutside the tube, the openings and the enclosures being so proportionedand arranged that waves of frequencies lying within a predeterminedrange are transmitted through the conduit without serious attenuationand that waves of frequencies lying outside such.

range are materially attenuated.

14. An acoustic wave filter, comprising a circular tube having openingsin spaced relation in the direction of its length and an acousticenclosure surrounding each opening outside the tube; the openings andthe enopenings and compartments being so propor-- tioned and arrangedthat waves of frequencies lying Within a predetermined range aretransmitted through the inner tubular body without serious attenuationand that waves of frequencies lying outside such range are materiallyattenuated. A

16. An acoustic wave filter, comprising two concentric tubes, aplurality of axially spaced partition walls defining annular interspacesand communications between the inner tube and the said spaces,saidcommunieations and interspaces being so proportioned and ar-' rangedthat waves of frequencies lying within a predetermined range aretransmitted through the inner tube without serious attenuation and wavesof frequencies outside said range are materially attenuated.

17. An acoustic wave filter, comprising two co-axially disposed tubularbodies, a plurality of axially spaced partition walls dividing theinterspace into separate compartments, openings between the innertubular body and the separate compartments and tubes extending throughthe outer tubular body into the'compartments into close proximity to theopenings in the inner tubular body in axial alignment with saidopenings.

18. An acoustic wave filter, comprising two concentric tubes, aplurality of axially spaced partition walls defining annularinterspaces, openings between the inner tube and the said spaces andtubes extending through the outer tube into the interspaces intoproximity to the openings in the inner tube and in-axial alignment withthe said openings.

19. The combination with a sound-produce ing device, of a wave filtercomprising an acoustic conduit including an impedance so proportionedand arranged that sound waves emitted by the producing device and offre-.

quencies lying within a predetermined range are transmitted withoutserious attenuation and that waves outside such range are materiallyattenuated.

20. The combination with a sound-responsive device, of a wave filtercomprising an acoustic conduit including an impedance so proportionedand arranged that waves passing to the sound-responsive device and offrequencies lying within apredetermined range are transmitted to thesound-responsive device without serious attenuation and that waves offrequencies outside such range are materially attenuated.

21. Anacoustic wave filter comprising a tubular body enclosing anacoustically conductive medium, means around the tubular bodyacoustically confining a plurality of axially spaced volumes ofanacoustically conductive medium and an acoustical communication betweenthe medium in the tubular body and each of the volumes around it.

22. In an acoustic filter, a sound-conducting path, and means'associated therewith for preventing the passagealong said path of soundwaves of a predetermined continuous range of frequencles.

23. In anacoustic filter, a sound conduct- 7 ing passage and meansassociated therewith for preventing the passage therefrom of sound wavesof a predetermined range of frequencies.

24. In an acoustic filter, a sound-conducting path, and means associatedtherewith for effecting a material attenuation of sound waves ofv apredetermined continuous range of frequencies While effectingsubstantially no attenuation of sound waves of another continuous rangeof frequencies.

25. In an acoustic filter, the combination with a sound-conducting path,of means located outside of and cooperating with said path forpreventing the passage along said path of sound waves of a predeterminedcontinuous range of frequencies.

26. In an acoustic filter, the combination with a sound-conducting path,of means located outside of and cooperating with said path forselectively effecting sound waves traversing said path by attenuating inmaterially different degrees sound waves of two different andnon-overlapping continuous ranges of frequencies.

In testimony whereof, I aflix my signature.

GEORGE WALTER STEWART.

